1. Field of the Invention
The present invention relates to an optical deflector for deflecting and sweeping a light beam such as a laser beam.
2. Description of the Related Art
Recently, an optical deflector, in which a mirror and piezoelectric actuators are formed integrally on a semiconductor substrate, has been proposed as a MEMS (micro electro mechanical systems) device using semiconductor processes and micro machining technology. In the optical deflector, one end of a piezoelectric actuator is linked to and supported by a frame part (a support part), and a torque, which is generated by an piezoelectric actuator, is transmitted to a torsion bar (an elastic beam) which is in turn connected to other end of the piezoelectric actuator, thereby rotarily driving a mirror attached to an end of the torsion bar. Advantages of such an optical deflector are: such an optical deflector is small in size, is simply structured, provides a large driving force, and is easy to be manufactured and mass-produced.
Because such an optical deflector has movable parts (torsion bars, piezoelectric actuators), the optical deflector may malfunction due to external vibrations, or be damaged by external impacts. One way to buffer the external vibrations or impacts is to cover a whole module that mounts the optical deflector with a cushioning material. But if one attempts to cover the module having the mounted optical deflector with a cushioning material, the overall size of the module becomes large, and the advantages of MEMS devices, such as being small in size and having high functionality, would be lost. For this reason, various techniques have been proposed to equip the optical deflector itself with a buffering mechanism that fights against vibrations and impacts. For example, movable parts of the optical deflector may be structured rigid and virtually indestructible. In another example, the optical deflector may be equipped with a buffering mechanism to absorb vibrations and impacts (See, e.g., Patent Documents 1-3). In the optical deflector in Patent Documents 1 and 2, a cross-sectional shape of a torsion bar is designed in such a way as to buffer vibrations and impacts. In the optical deflector in Patent Document 3, a cushioning material is directly glued to a fixing frame which touches the bottom side of the device.    (Patent Document 1) Japanese Laid-open Patent Application: Publication No.: 2004-034256.    (Patent Document 2) Japanese Laid-open Patent Application: Publication No.: 2004-264702.    (Patent Document 3) Japanese Laid-open Patent Application: Publication No.: 2004-347713.
But if the torsion bar is structured rigid, the rigidity of the torsion bar increases, resulting in a reduced maximum deflection angle. On the other hand, if the cross-sectional shape of the torsion bar is designed to be less prone to a destruction as in the cases of Patent Document Nos. 1 and 2, the number of manufacturing steps is increased in order to form a more complex structure, and the yield is reduced due to the required higher precision manufacturing.
In the optical deflector of Patent Documents 3, a cushioning material is glued to the bottom surface of a fixing frame. However, because the optical deflector has movable parts, the area of the fixing frame (an area for gluing the cushioning material) relative to the entire bottom surface of the device is limited, and therefore, it is difficult to obtain sufficient cushioning effects against the external vibrations and impacts. Moreover, in optical deflectors, the mirror is often damaged by a collision of the mirror to the fixing frame when the external vibrations and impacts are transmitted to the mirror through the torsion bars to cause abnormal vibration in the mirror. In the optical deflector of the Patent Documents 3, it is difficult to avoid this type of damage of the mirror by the collision with the fixing frame when the mirror is vibrated abnormally.
Furthermore, in silicon based MEMS devices, because of the large difference between the thermal expansion coefficients of the silicon and the cushioning material (such as rubber), a large stress is generated on the interface depending on the surrounding temperature. Therefore, in the optical deflector which has the glued cushioning material as in the Patent Documents 3, the deflection and sweeping/scanning performance of the deflection device may suffer due to the stress which is caused by the cushioning material.